In the course of handling crude oil and refined petroleum products, the small percentage of residues which are present accumulate in storage holding areas because with time in storage such residues separate from the basic crude oil or the refined petroleum. The amounts of these residues that accumulate depends on the crude oil or refined petroleum being stored. Complicating this condition is the fact that in one way or another, water and siliceous materials are introduced to the holding areas and accumulate with the residues. These residues have fuel value. However, gaining access to them within the holding areas is difficult until the holding area is free of its normal storage, and even then, the recovery of the residues is a problem. In the past, after the area was free of the normal storage, crews were sent into the area and they shoveled the residues out. Vacuum suction has been used to remove the separate layer of water either before or after the work crews entered the area. Because the resolution of this problem was so labour intensive and hazardous, and carried out irregularly, there has been a lessened inclination to clean the storage holding areas, consequently many of them have large accumulations of such residues and water. This has introduced a massive problem for the refiner which involves serious economic and environmental penalties.
Owing to an inability to recover these residues effectively and economically and to render them useful as fuels, residues of crude oil and/or heavy fuel oil, and the like, have low commercial value. They commonly have high viscosities, and contain, among other things, insoluble carbonaceous particulate matter, sand, other inorganic particulate materials and/or water. As a result, they have been discarded into pits or ponds which over time have become serious environmental problems and imposed significant problems in land utilization.
The complexity of the problem deserves a more thorough discussion. Crude oils, heavy fuel oils, and the like, are typically stored in holding tanks having a capacity of from about 2.5.times.10.sup.5 to 15.times.10.sup.6 gallons or more. They may be left in the tank for weeks at a time, consequently insoluble residues have ample opportunity to precipitate within the oil in the tank and settle to the bottom of the tank where the insoluble residues may become assimilated with any water layer present..sup.1 With time, the volume occupied by these residues (and sludges) within the storage tank becomes appreciable. This volume will continue to build with each succeeding charge of oil into the storage tank thereby reducing the storage volume of the tank for the desirable crude oils and heavy fuel oils. FNT 1. Water has a higher specific gravity than oil and settles to the bottom of the tank.
Eventually, either to maximize and restore the holding capacity of the tank or to empty the tank for purposes of inspection or repair, and the like considerations, these residues (sludges) have to be removed from the tank. As mentioned earlier, the problem had been met by workers entering the tank through its manways or an upper opening (e.g., top cover), and proceeding to shovel the sludge out of the tank. Not only is this primitive technique labour intensive, and time consuming, resulting in an inordinate amount of downtime for the tank, it also creates serious health and environmental problems. Other sludge removal techniques have been developed including, for example, vacuum suction utilizing negative pressure, dilution with a solvent such as light gas oil/distillate, and the like. While these techniques are perhaps improvements over manual recovery of residues from tanks, they are expensive and still pose health, safety and ecological problems. They give little thought to recovering and treating the removed residues in an economical and efficient manner. In addition, the use of solvents adds a significant cost since the solvent has value in commerce.
The residues shoveled or otherwise taken from the tanks have been carted in batch operations from the tank storage areas to large excavated holes in the ground where they are deposited to create pits or ponds of such residues. These residues eventually transform into pitch. With time, the pits or ponds have grown into substantial environmental headaches for the refiners and their purlieus.
As the value of petroleum has increased in the past decade, coupled with recognition that the accumulation of residues is a problem that will not go away, and has to be dealt with, more interest has been taken in the energy values of the residues because only in the effective utilization of the residues as a fuel or raw material can the environment be cleaned up. Key to energy value attractiveness of these residues are two factors:
However, inasmuch as access to these tanks is generally accomplished by means of the manways, which are typically located at the lower portions of the side(s) of the tanks, residue removal techniques, regardless of the specific procedure employed, have generally been carried out on a frequent enough time interval so as to prevent the height of the accumulating residue material within the tank from reaching a level which is higher than the height of the manway location which would, of course, present serious problems in gaining access to the tank and the contained residues.
A need accordingly exists for a process which provides an economical and efficient means for removing crude oil and/or heavy fuel oil residues, and the like, from a storage tank in a safe and ecologically sound manner and which, moreover, also provides for the recovery of such removed residues so that they can be economically utilized.